1. Field of the Invention
The invention relates to a display device comprising a display tube having at least one cathode, and a black level setting circuit for setting the black level of a video signal to be applied to the cathode, said black level setting circuit comprising a correction circuit for correcting the black level and for supplying a correction value for the cathode, control means for applying a predetermined control signal to the cathode during a measuring interval, and a measuring circuit for measuring a current generated in the cathode by the control signal, said measuring circuit having an output which is coupled to an input of the correction circuit.
At a voltage of 0 V at the cathode (with respect to the first control grid), a given current flows in the cathode. If the voltage increases, the current decreases until the current becomes zero. This is referred to as the cut-off point. The problem occurring when controlling the display tube is that the control is to be effected from the cut-off point. The cut-off point of a display tube is difficult to determine, certainly if this is to be effected automatically. If there is no control voltage, the voltage at the cathode must be equal to the cut-off voltage, thus just precluding the flow of beam current. The black level setting circuit is used to render the black level equal to the cut-off voltage.
2. Description of the Related Art
A display device of this type is known from Valvo Technische Information of Jul. 19, 1985 "Die Video-Kombination TDA 4580". At the black level setting circuit in this known display device, a fixed control voltage is applied as a control signal to the relevant cathode for each color during one measuring line in the field retrace interval. The cathode current Ik is measured and compared with a reference current Iref. The black level is corrected until the cathode current is equal to the reference current. In this way the black level is set close to the cut-off voltage.
A drawback of this known display device is that the black level of the amplifier never corresponds exactly to the cut-off point of the display tube.
The invention is based on the recognition that the relation between the cathode, current Ik and the control voltage V.sub.D (i.e. the voltage applied to the cathode, sent in a negative direction from the cut-off point) is approximately as follows: ##EQU1## The cathode current Ik is dependent on the cut-off voltage Vco and on .gamma.. .gamma. is a quantity related to the display tube. Among the different cathodes for the colors (R, G, B) in one display tube, the cut-off voltage may have different values varying, for example between 105 and 130 Volts. One reference current for one display tube is thus insufficient. Moreover, the cut-off points also vary due to heating, ageing, etc. The reference current should then vary accordingly.
In the above-mentioned black level setting circuit, .gamma. is required as an input parameter and will therefore have to be measured and taken into account for each type of display tube.